$600 return is a very good deal. If I ever saw that from SF or even LA, I'd grab it instantly.
To be clear, this is for BsAs - Miami - BsAs, not vice versa.
The origin of the trip matter a great deal on routes like this.
By the way, LA also has great rates for late February (return must be in mid-March).
765 direct on American, or 650 with LATAM with stops.
<shameless plug>
Rather than futz around doing trial-and-error online, it's easier to speak with a professional who will give you some insight on, say, restrictions imposed by the fare. For example, for the aforementioned price of 765 to LA on American, you could just play around on Kayak for awhile, or you could have someone tip you off to the fare conditions:
The moral being that you often save yourself time, and increase your chances of getting the best price possible, when going through a professional as opposed to going it alone. Especially when using the agent often costs no more than the price online.
</shameless plug>
